Mr. de Luc 
4°4 
own, and consequently it is proportional to its own power ; 
and in open air, the part of the whole pressure incumbent 
on steam is, to that whole, as its power is to that of the whole 
mass, the rest of the pressure being supported by the air 
with which it is mixed : which proportion in the pressure 
that steam undergoes, in this case, comes exactly to that of the 
first. This is agreeable to the laws of expansible fluids, and 
the following example, taken at a determined temperature, 
will prove it from experience. 
9. The thermometer being at about 65° of Fahrenheit, the 
maximum of evaporation, in an exhausted receiver, keeps a 
column of quicksilver of 0,5 inch suspended in the short ma- 
nometer (I have quoted the experiments which give that 
middle result). That phenomenon has been considered by 
some natural philosophers as distinct from common evapo- 
ration, and depending on a disposition of the particles of 
liquids of repelling each other ; which disposition, supposed 
to be counteracted by the pressure of the atmosphere, must 
appear when that pressure is removed. But, besides the 
known loss of heat by the liquid, in this case as well as in 
open air, which points out the same cause of evaporation, 
an equal pressure is also produced, and added to that of air, 
when the receiver is filled with the latter ; which again in** 
dicates the same effect, as will appear in the following ex- 
ample. 
10. If, by the above temperature, the receiver is filled with 
air of the same density as the air of the place ; in which 
case a barometer inclosed in that receiver will stand at the 
same height as in the open air ; and that in the receiver, be- 
ing then very dry, water be introduced in a quantity sufficient 
